The present invention generally relates to printers and in particular to a system and method for using pulse or trickle warming to control neutral color balance on a print media.
Inkjet printers print dots by ejecting very small drops of ink onto the print media and typically include a movable carriage that supports one or more print cartridges each having a printhead with a nozzle member having ink ejecting nozzles. In general, the ink is housed in a vaporization chamber with a tube leading to a nozzle exposed to the print media. Small drops of ink are ejected from the nozzles through orifices by rapidly heating a small volume of ink located in the vaporization chambers with small electric heaters, such as small thin film resistors.
Gas is held in solution in liquids such as ink. The colder the ink, the greater the amount of gas that is held. As the ink increases in temperature, the solubility of the gas decreases, and it leaves the solution in the form of bubbles. The higher the temperature, the more bubbles are formed, and they form at a faster rate. If the temperature reaches a sufficiently high temperature the solution itself may reach its boiling point and also form a gas. The bubbles from either source choke the nozzles and cause deterioration in the quality of the image on the print media.
Temperature also controls the uniformity of the drop size of the ejected ink. The heat from the resistors causing the explosive vaporization in the chamber also causes the size of the drop of ink formed in the chamber to vary. There is an optimal temperature operating range for printheads using inks. If the temperature is too low the ink droplets formed will be smaller and have a lower drop-weight than that required for good image quality. As the temperature rises, the drop-weight of the ink droplet will rise. The variation in drop weight varies with the ink being used. These variations in drop-weight will cause visible color shifts in the printed image.
The temperature will be high if the resistors fire a number of times in a short period of time. Also, if the length of the current pulse to the resistor is longer than a pre-determined limit. As the carriage traverses in a print swath, various heater elements in the array are activated. If the traverse is narrow, the mean temperature at the beginning of the traverse will be similar to the mean temperature at the conclusion, and the effect of temperature on the pass will be consistent for all ink droplets projected onto the print media. If the swath is wide, and more heater elements are activated, the mean temperature at the end of the pass may be considerably higher than at the beginning. The difference in temperature from the beginning of the pass to the end of the pass could result in variation in the drop-weight of ink droplets on the same pass. This would result in color variation on the one line of print.
Generally, the temperature of the printhead is approximated by two measurements, the thermal sense resistor (TSR), and the digital temperature sensor (DTS). The DTS is a point sensor located at the top of the die near a firing heating element. While this sensor more accurately reflects the temperature at that point, it is not an accurate measure for other heating elements on the die.
The TSR is an approximation of the mean temperature of the printhead die. It is not located adjacent to any particular heating element and reflects the temperature of the die after heat has moved from the heating elements to the TSR. There is, therefore, a delay in the temperature reported by the TSR. The longer the printhead fires, the greater will be the temperature recorded by the TSR. When the printhead has been idle, for example, at the beginning of a print pass, the temperature recorded by the TSR will be low as the die will be cool. The droplets produced at this time will be of low drop-weight. As the pass continues and the number of heating elements firing has increased, the temperature at the TSR will have increased and the drop-weight of the ink droplets will have increased. The difference in temperature from the beginning of the pass till the end of the pass will affect the size of the ink droplets across the pass.
To minimize the effect of temperature variance from the beginning of printing to another point in the printing process, a warming device may be employed. A warming device is used to raise the temperature of the printhead. The printhead assembly may include a means to control the electrical current to the firing resistors so that their temperature is below the threshold required to eject an ink drop. This device could be a power field effect transistor (FET). The device provides a capability to warm the printhead assembly to the desired temperature before or during printing operations. The process is called xe2x80x9ctrickle warmingxe2x80x9d because the printhead assembly allows only a trickle of energy to flow through separate FETs to firing resistors. The printhead assembly temperature rises until the desired temperature is reached and the warming device is then shut off. Other embodiments of the invention may employ a pulse warming system to effect a similar result.
However, these systems are problematic because they do not incorporate changes in either ambient temperature or ambient humidity in calculating the preferred droplet size to be printed to the print media. Ambient temperature and ambient humidity have an effect on the print media so that the absolute color consistency of printed images are affected by their change. The ink interacts with the print media, and if the physical nature of the print media has been altered by changes in the ambient conditions, then the interaction of inks with the print media will vary with each change. Therefore, what is needed is a system and method that overcomes these problems.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention includes an embodiment for optimizing the temperature operating range for a thermal inkjet printhead using ink over large print swaths.
In general, this embodiment includes receiving the temperature of a digital temperature sensor (DTS) before printing begins, comparing this temperature with the set point for printing, initiating heating elements if the temperature is below the printing threshold, and turning off those heating elements when the threshold temperature of the die has been reached.
The method and process minimizes thermal excursions, either above or below the set point for the production of ink droplets, and maintains an approximate isothermal environment for operation. This in turn would lead to ink droplets of a consistent drop weight and would further lead to a consistent production of color. In the description that follows, color is referred to tone, hue and chroma. In one example, the system and method of the present invention can be implemented in a black and white system where the tone range is modified.